The electrophoretic profiles of serum insulin-like growth factor binding proteins (IGFBPs) from an 18-year-old patient with growth hormone (GH) insensitivity were analysed by Western ligand blotting before and after administration of recombinant human insulin-like growth factor I (IGF-I). Under basal conditions, the profile was the same as that observed in all cases of GH deficiency or insensitivity, that is, low IGFBP-3 (seen as two bands of 41.5 and 38.5 kDa) and enhanced IGFBP-2 and, to a lesser extent, IGFBP-1. Subcutaneous injection of IGF-I, 40 micrograms/kg, provoked an increase in serum IGF-I levels to close to the lower limits of the normal range and a small increase in IGFBP-3, suggesting that IGF-I has a direct effect on the synthesis of this GH/IGF-I-dependent IGFBP. Moderate increases were also observed in IGFBP-2 and IGFBP-4. Repeated doses of IGF-I over 7 days had no further effect on these changes, which persisted for a few days after the last injection.